On Tue, 18 Mar 2003 12:33:44 +0100
Daan Leijen
I disagree with this design. It is very awkward to install callbacks this way as we have to maintain the "unregister" function. We can no longer use "nice" syntax like:
button [on click := messageBox "hi there"]
You can have the "unregister onClick action" in the resulting "ButtonType" value, just as you keep there other important attributes of a button.
I also think that "register/unregister" is a confusing name. Here is my proposal. We have two functions for each event type. One to set the current callback, and one to get the current callback. (there is only one callback at any time).
Yes, but: if you want to modify the current callback, for example to append another with ">>", you have to lock it if there are other haskell threads; you can't just get it, modify and put back, you need a "modifyCallback" action. This can be implemented with MVars of course. The other way around, wich is "registerCallback :: (a -> IO b) -> IO (IO ())" is more useful if one allows more than one callback at a time. It seems to me that one thing is implementable by the other, by tricking with functional equality, so I guess we will choose the one that matches more toolkits. Vincenzo